Since its discovery in 1965 the cosmic microwave background radiation has been one of the pillars of the Big
Bang model. (1) Measurements of its spectrum firmly established the hot big bang model as the basis of our
understanding of cosmology. (2) Measurements of the anisotropy of the CMB over the last fifteen years have
enabled us to determine cosmological parameters such as the age, density and composition of the universe (3, 4) to
unprecedented accuracy. Recent measurements of the polarization of the CMB provide strong support for our
model of the CMB. (5, 6)

As a result of these and other astrophysical observations we have a very successful model for how structures
have formed in our universe. Gravitational instability makes fluctuations grow with time, but the fluctuations
require initial seeds. The CMB temperature power spectrum and the angular dependence of the cross-correlation
between temperature and polarization7 imply that these seeds had to be created very early in the evolution of
the universe. (8–10)

The leading scenario for the creation of the primordial seeds is the paradigm of inflation. (11–14) The fluctuations
we see today are the direct result of quantum fluctuations of a very light scalar field, the inflaton, during a period
of accelerated expansion or “inflation”, a small fraction of a second after the big bang. (15–21) However, many
of the details of the inflationary scenario are uncertain and perhaps more importantly, the paradigm currently
lacks any strong confirmation.

A promising way to confirm the inflationary scenario is via its prediction of a stochastic background of gravity
waves, (18, 19, 22–24) which we call the inflationary gravitational-wave background (IGB). The best known way to
search for the IGB is through its signature on the CMB polarization. (25, 26) Thomson scattering of an anisotropic
radiation field leads to linear polarization. The temperature anisotropy produced by density perturbations and
by gravity waves both lead to linear polarization through this mechanism. However, the pattern of the induced
polarization on the sky should be very different in each case. Density perturbations produce a curl-free or E-mode
pattern of polarization vectors. Gravity waves produce a curl or B-mode pattern of polarization vectors that
density perturbations cannot produce. (27, 28) Thus if polarization can be mapped and decomposed into E and B
components, the B component will be an unambiguous signature of the IGB and therefore
for inflation. Detection of this IGB signal is a primary goal of the EBEX experiment.

The IGB signal is extremely faint – with a similar or smaller amplitude than expected foreground signals. It
is anticipated that the most significant foreground signal will be from polarized Galactic dust29. However,
there is currently very little information about the level of polarized dust and its orientation as
a function of position on the sky, or about its E and B power spectra. No information exists for any region of the
sky at the accuracies required for a B-mode signal detection. A primary goal of EBEX will be to characterize
the polarized dust emission and determine its angular power spectra in both E and B-type polarizations.
The dust signal increases with increasing frequency, while the CMB signal is at a maximum at 150 GHz. The
measurements at 250, 350, and 450 GHz will be primarily sensitive to dust emission, and by extrapolating these
measurements to 150 GHz the dust foreground will be subtracted from the primary CMB band at 150 GHz.

The predicted uncertainty in the determination of the dust signal at 150 GHz is shown in the right panel of
Fig. 1. The dust measurements will also provide the community with critical information about the polarized
dust fraction and orientation as a function of frequency. Given the expected magnitude of the polarized dust
foreground, this information is essential for all future CMB polarization experiments.

We have chosen a balloon-borne platform for EBEX because this allows measurements over a broad frequency
range and reduces atmospheric effects by three orders of magnitude. (32) At frequencies higher than 150 GHz
atmospheric emission becomes significant and broad-band observations from the ground are difficult because
of a combination of lower detector sensitivity (a result of increased incident power) and higher noise (due to
fluctuations in sky emission). Broad-band, ground-based observations are essentially impossible from anywhere
on Earth at the higher frequency bands. EBEX is unique among CMB polarization experiments in that it will
have four frequency bands between 150 and 450 GHz. This is the broadest frequency coverage of all current and
proposed bolometric CMB polarimeters and gives EBEX an unprecedented capability to measure the polarization
of the dust emission.